The present invention relates to a method and device for setting the braking action on the wheels of a motor vehicle.
In conventional antilock control systems or driving stability control systems, such as those described in, for example, the article xe2x80x9cFDRxe2x80x94die Fahrdynamikregelung von Boschxe2x80x9d [Driving dynamics control by Bosch], ATZ Automobiltechnische Zeitschrift 96 (1994), pp. 674-688, the braking effect on the vehicle wheels is set on the basis of the driving stability, independently of the driver. In antilock control systems this prevents the vehicle wheels from having an excessive slip, whereby the possibility of lateral forces being built up by the wheels is reduced. On roadways having a loose surface (gravel, rocks), however, the shortest braking distance can be achieved with the wheels locked, since in this case a certain chocking effect occurs. In addition, on such roadways and also on extremely poor stretches of road, very frequent and very extreme changes in wheel slip values occur. The vehicle wheels often seem to tend to lock since, due to the roadway unevenness, the wheels are caused to be considerably braked by the loose road surface. This results in an antilock control system causing too frequent or excessively strong braking pressure reductions on the average, which may result in an underbraked vehicle behavior.
Furthermore, on an uneven roadway, situations where the wheels are apparently unstable, and thus tending to lock, may occur for a short period of time even in the event of partial braking with a low braking pressure due to fluctuations in the wheel support forces, which, without additional measures, results in antilock control cycles being unnecessarily triggered. This results in reduced comfort, for example, due to pulsating brake pedal, and in unnecessary load on the antilock control system hydraulics, for example, due to switching of the brake pressure valves and activation of the recycling pump.
European Patent 495 796 B1 describes an antilock control system and an antislip control system which optimizes braking on uphill and downhill slopes. In order to determine the uphill or downhill roadway grade, the longitudinal vehicle acceleration is measured by acceleration sensors and compared with a longitudinal acceleration value determined from the rotational speed of the wheels. If an uphill or downhill grade is present, the antilock control system is made less sensitive, for example, by increasing the slip threshold or the deceleration threshold.
The antilock control system described in World Patent No. 90/06870, shows that the sensitivity of the control depends on vehicle deceleration, and the antilock control system, in particular the slip control system, is made less sensitive in response to the detection of controlled downhill.
One object of the present invention is to achieve optimum setting of braking action on roadways with a loose road surface (rocks, gravel or the like).
The present invention relates to a method and to a device for setting the braking action on the wheels of a motor vehicle having at least two wheels arranged on a rear axle and two wheels on a front axle. A wheel quantity representing the stability of the wheel is formed at least as a function of the wheel slip. This wheel quantity (as a first comparison quantity) is compared with a corresponding threshold value (as a second comparison quantity). The braking action is set as a function of the result of this comparison.
A friction value which represents the friction coefficient on one wheel or on a plurality of wheels of the rear axle is determined. At least one of the comparison values (friction value and/or threshold value) on one wheel or on a plurality of wheels of the front axle is modified as a function of this friction coefficient which describes the friction conditions of the rear wheels. This has the advantage that the determination of the friction value on the rear wheels is not influenced by modifications of the front axle wheels.
In an advantageous embodiment of the present invention, a grade value is determined which represents the longitudinal grade of the roadway in the direction of travel on which the vehicle travels (uphill or downhill grade). Depending on this grade value, at least one of the comparison values is then modified on at least one wheel of the front axle. In this embodiment of the present invention, the control on the front axle is effected as a function of the roadway grade via the friction coefficient. In particular, in the event that a downhill grade of the roadway is detected off road, the comparison values (slip and/or slip threshold value) are subject to a particularly high degree of modification. This makes it possible to lock one or both front wheels, which, in the case of a loose roadway surface, results in drastic improvement in the braking action.
In an example embodiment of the present invention, the wheel rotational speed quantities, which represent the rotational movement of the wheels, are detected. A longitudinal acceleration value representing the actual vehicle acceleration is determined from these rotational speed quantities. Furthermore, an estimated value of the longitudinal acceleration of the vehicle is determined at least as a function of quantities representing the braking and/or driving action on the wheels. Such quantities are understood in particular as the admission pressure determined by the actuation of the brake pedal by the driver, the brake pressure valves"" opening times, or the output torque acting as the driving torque on the wheels. According to this embodiment of the present invention, the grade value is determined as a function of the longitudinal acceleration value and the estimated value. This is done by comparing the actual longitudinal acceleration value and the estimated value. The grade value is determined according to the present invention, contrary to European Patent 495 796 B1 mentioned above, without an additional acceleration sensor.
In another example embodiment of the present invention, at least one longitudinal force value representing the longitudinal force acting on at least one wheel is determined. In addition, a normal force value representing the normal force acting on at least one wheel is determined. Normal force is understood as the force acting perpendicularly to the roadway between the wheel, i.e., the tires, and the roadway. In this embodiment of the present invention, the friction value is determined as a function of the longitudinal force value and the normal force value.
In an example embodiment of the present invention, a steering quantity representing the steering angle of the vehicle is determined. This can be, for example, the steering angle of the steering wheel actuatable by the driver. Then, according to the present invention, at least one of the comparison quantities on at least one wheel of the front axle is modified as a function of the steering quantity. In particular, if a steering intention of the driver is detected, in order to preserve steerability, the modification according to the present invention is not implemented or the modification according to the present invention is implemented to a slight degree compared to the case where no steering intention is detected.
When greater friction coefficients are present, which is detected in particular by the fact that a predefinable threshold value of the friction value is exceeded, the modification according to the present invention is implemented on both wheels of the front axle. If, however, only smaller friction coefficients are present, which is detected by the fact that the friction value is less than a predefinable threshold value, the modification according to the present invention is only implemented on a single wheel of the front axle. When a very small friction coefficient is present, which is detected, in particular, by the fact that the friction value is below a predefinable very low threshold value, no modification is implemented on the front axle wheels according to the present invention.
When smaller friction coefficients are present, which is detected by the fact that the friction coefficient is below a predefinable threshold value, the modification according to the present invention can be implemented on a single wheel of the front axle so that the modification is performed alternatingly on the two wheels of the front axle.
The embodiments described in the two previous paragraphs have the advantage that the modification according to the present invention can be adapted very accurately to the prevailing friction conditions. If a higher friction value is detected, the modification is allowed on both front wheels, which results in increased tendency of both front wheels to lock.
However if smaller friction coefficients are present, the modification according to the present invention is only implemented on one front wheel, which results in a shorter braking path, while sufficient steerability and stability are still guaranteed. Only in cases of very low friction coefficients, for example on ice, is maintaining steerability a priority; therefore, in those situations, the modification according to the present invention is disabled and does not take place.
In another example embodiment of the present invention, a velocity value representing the vehicle""s longitudinal velocity is determined. Such a velocity value is derived from the wheel rotational speed as a reference velocity. The modification according to the present invention is only implemented in the case of lower velocity values, which is detected, in particular, by the fact that the velocity value is less than a predefinable velocity threshold. This embodiment is based on the fact that in general relatively low longitudinal vehicle velocities occur when traveling off road, i.e., at sufficiently low longitudinal vehicle velocities, shortening the braking path has priority over steerability.
Furthermore, a roadway value representing the roadway unevenness can be determined. The determination of such a roadway value is described in detail in the literature as roadway unevenness recognition. The modification according to the present invention is effected as a function of the roadway value. This is accomplished by the modification (an increase in the tendency of the front wheels to lock), in the case of roadways having a high degree of unevenness, being more pronounced than in the case of roadways having a lower degree of unevenness.
In general, a transmission is arranged in the power train of the vehicle between the vehicle engine and the vehicle wheels. In off-road vehicles the transmission may have an off-road gear (low gear) selectable by the driver. The modification according to the present invention depends, in an advantageous embodiment of the present invention, on the selection of the low gear. This means that for vehicles having an off-road low gear setting, the fact that the low gear has been set is used as an additional criterion for carrying out the modification according to the present invention. Thus, for example, the modification according to the present invention (off-road measures) can be disabled in high gear. In addition, the above-mentioned threshold values for detecting an off-road situation can be configured differently in high and low gear.